Also, rename lockContent to lockContentExclusive
inAnnexSafe should perhaps be eliminated, and instead use
`lockContentShared inAnnex`. However, I'm waiting on that, as there are
only 2 call sites for inAnnexSafe and it's fiddly.
This fixed one bug where it needed to be and wasn't (in Assistant.Unused).
And also found one place where lockContent was used unnecessarily (by
drop --from remote).
A few other places like uninit probably don't really need to lockContent,
but it doesn't hurt to do call it anyway.
This commit was sponsored by David Wagner.
